Electric furnace dust generated from a steelmaking electric furnace is dust in ultrafine powder form, which contains not only iron but also nonferrous metal, such as zinc and lead. Heretofore, with a view to effectively utilizing such electric furnace dust, various treatment methods have been proposed.
For example, there has been proposed a technique of adding a carbonaceous material to electric furnace dust, granulating the mixture with water into pellets, heating the pellets in a rotary kiln to collect zinc and lead through reduction and volatilization thereof, and collect iron in the form of reduced iron pellets through reduction thereof (see, for example, Patent Document 1). The pelletization of electric furnace dust with addition of a carbonaceous material thereto offers the following effects.
(1) The carbonaceous material contained in the pellets promotes the reduction and volatilization of zinc and lead, and the reduction of iron.
(2) Although a rotary kiln is generally installed in a location away from an electric furnace, scattering of a fine powder during transportation can be prevented by pre-pelletizing electric furnace dust near the electric furnace to transport it to the rotary kiln in the form of a pellet.
However, the above pellet is easily broken/pulverized by shock arising from tumbling in the rotary kiln. This causes problems such as formation of deposit on a furnace wall or occurrence of a large amount of dust loss.
In view of such problems, there has been proposed a technique using a rotary hearth furnace in place of the rotary kiln to heat and reduce pellets on a hearth rotated in a horizontal plane (see, for example, Patent Document 2). This technique makes it possible to collect zinc and lead while collecting iron in the form of reduced iron pellets, without breakage/pulverization of the pellets.
However, this technique, in the same manner as that in the technique using the rotary kiln, requires addition of a carbonaceous material and a large amount of water to electric furnace dust for palletizing it, which raises a water content in a pellet (raw pellet) to 7 to 15 mass %. Although the Patent Document 2 discloses a direct charge of raw pellets into a rotary hearth furnace without drying, to heat them (the paragraph [0013], FIG. 2 in the Patent Document 2), the sudden charge of raw pellets containing such a large amount of water into a high-temperature atmosphere involves a rapid application of heat to the pellets to cause a rapid vaporization of the water in the pellets, thus generating a gas pressure of the water which is likely to cause bursting of the pellets.
In order to avoid the bursting of the pellets, there has been proposed a technique of providing a preheating zone in a preceding stage relative to a heating zone of the rotary hearth furnace, and preheating the pellets in the preheating zone at a temperature in a relative low region to slowly remove water from the pellets while preventing the occurrence of bursting (see Patent Document 3). However, this technique involves a significant lowering in treatment rate of electric furnace dust when a size of the rotary hearth furnace is maintained, or involves a significant increase in size of the rotary hearth furnace when the treatment rate of electric furnace dust is ensured.
It can be also contemplated to charge raw pellets into a rotary hearth furnace after drying the raw pellets using a drying machine to reduce a water content in each of the pellets to 1 mass % or less (see Patent Document 4). The charge of the sufficiently dried pellets into the rotary hearth furnace ensures the prevention of the bursting and the sufficient treatment rate of electric furnace dust while maintaining the size of the rotary hearth furnace; however, it requires an additional drying machine and large energy for the drying.
In view of the above problems, the applicant has proposed, in place of the technique of pelletizing a powdery mixture comprising electric furnace dust and a carbonaceous material, a technique of briquetting the powdery mixture through a compacting process, and then charging the briquettes in a rotary hearth furnace (see Patent Document 5).
The briquetting based on a compacting process, which does not generally require a large amount of water as compared with the pelletization based on tumbling granulation, can offer expectation of omitting the drying machine and drastically reducing energy necessary for drying of the briquettes.
However, there is unknown a briquette production condition (particularly, about an adequate water content) for reliably preventing briquettes from bursting thereof in a rotary hearth furnace while giving the briquettes a enough strength to resist a handling thereof after compacting through until charging into the rotary hearth furnace.    [Patent Document 1] JP 6-174383A    [Patent Document 2] JP 9-209047A    [Patent Document 3] JP 11-337264A    [Patent Document 4] JP 11-193423A    [Patent Document 5] JP 2004-76152A